Tabernacles, compression posts and partners oh my!

  • 25 Feb 2020 20:31
    Reply # 8769998 on 8753316

    Hi Adam,

    I think using a Bermudan compression post below the partners of a Junk Rig mast is not necessarily problem-free.  Below a Bermudan mast, the compression post will take up only axial compression load, and it needs a certain diameter to avoid buckling (sideways flipping-out) under the compression load.

    However, unlike a Bermudan mast, an unstayed Junk Rig mast takes up a bending moment that is at its maximum at the partners, and in order to have enough bending strength it needs a larger diameter there than a Bermudan mast’s compression post.    I  From the step, the moment increases linearly from zero at the step to Mmax at the partners like I have tried to illustrate that by sketching on the photo..  At point “1” at the tips of the stiffeners, the moment M = M1 = Mmax * b/a.  By measuring on the photo, b/a = (a little above) 0,7, so the bending moment M1 will be about Mmax * 0,7.

    As the diameter of the compression post is that much smaller than the mast diameter at the partners, I am quite sure that the bending stress is much higher at “1” than at the partners.  I would therefore recommend that you check the bending stress at point “1”.  If it is too high, the solution is simply to extend the large diameter further downward until the stress level at the diameter change is comfortable.

    Good luck with your project!

    Nils


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  • 20 Feb 2020 22:00
    Reply # 8760351 on 8753650
    Anonymous wrote:

    Malcolm is doing something similar on SY Befur, but it involves a steel frame that goes right across the boat, and forwards as well.

    https://sy-befur.co.uk/2019/08/09/a-start-on-befurs-sailing-rig/

    Thanks for the reference David

    ...but then I have a reputation for over-engineering things to maintain!

    ...we have progressed to the point that we have a sail, courtesy of my better half Louise, (see attached pic) and a mast (installed here) and a lot of hardware and rope, we just need to assemble it by 1st April and tow it all to Windermere!

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  • 18 Feb 2020 16:33
    Reply # 8754367 on 8753400
    Anonymous wrote:

    Hi Adam,

    the problem is not the compression post/ tabernacle, but the deck strength. The freestanding mast will put forces into the deck that it was never designed for. My suggestion would be to look at cutting out part of the top layer of the deck and remove the balsa core, then lay up a solid fiberglass laminate that will transfer the loads back into the deck and hull. A lot more information would be required to design something that would work. You might also want to look at a keel stepped mast with a hinge above deck and a sliding pipe collar that gives it its strength. I have installed this arrangement on my boat Gypsy Rose and it is working quite well. I am intending to do an article on it for the magazine but not got to it so far.

    All the best with the project.

    David.

     very much appreciate the insight! I will look forward to seeing your work on Gypsie Rose!
  • 18 Feb 2020 05:30
    Reply # 8753400 on 8753316

    Hi Adam,

    the problem is not the compression post/ tabernacle, but the deck strength. The freestanding mast will put forces into the deck that it was never designed for. My suggestion would be to look at cutting out part of the top layer of the deck and remove the balsa core, then lay up a solid fiberglass laminate that will transfer the loads back into the deck and hull. A lot more information would be required to design something that would work. You might also want to look at a keel stepped mast with a hinge above deck and a sliding pipe collar that gives it its strength. I have installed this arrangement on my boat Gypsy Rose and it is working quite well. I am intending to do an article on it for the magazine but not got to it so far.

    All the best with the project.

    David.

  • 18 Feb 2020 04:08
    Message # 8753316

    I may be biting off a big bite here - but here it goes (and thank you in advance).

    Boat: C&C25 Mk I. Currently rigged as a Bermudan. 

    I've decided, for the sake of interior space, to keel step my mast via a compression post. To make matters even more complex I'm also intending to put the mast in a tabernacle. Hence the desire to reduce space taken up by the arrangement.  (Yes I am a glutton for punishment.)

    Having figured out that the best location for the mast is forward of my forward hatch, I'm confronted with a curving coach roof - see the attached diagram for an exaggerated doodle of what I'm aiming for. She's a cored GRP hull - core is balsa. 

    Long term the tabernacle is desired because my route out of Lake Ontario takes me through the Oshwego Canal which has two low, permanent bridges within the first 20NM. My air draft with the mast up will allow me to do the rest of the Intercoastal route down to Florida with the rig up and possibly saving the need to motor for parts of the journey. 

    Short term - my home marina requires us to unstep the masts at the end of the season - which I'd like to be able to do without the need to resort to lifting the mast out of the hull with a crane. Call me lazy, but I just loathe the faffing around. 

    My question(s) are as follows:
    1. can anyone recommend a good source on how the partners should be arranged and 

    2. how difficult is what I'm proposing. I haven't seen any examples of tabernacles married to compression posts for unstayed masts. Have I missed a trick? Some vital engineering element that has escaped me?


    [edit - I've added a photo of the boat as she is (more or less) now. The cabin top is sloped and curved for most of the foredeck - and in particular that area in front of the hatch]

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    Last modified: 18 Feb 2020 04:20 | Anonymous member
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